The present invention relates to a printed wiring board, and more particularly, to a multilayer printed wiring board composed of a laminate of conductor layers.
Currently prevailing are double-sided printed wiring boards, multilayer printed wiring boards, etc., which are provided with a plurality of conductor layers such that they can be reduced in size and improved in packing density.
In the printed wiring boards of this type, the conductor layers include, for example, a plurality of signal layers formed by patterning copper leaf and a ground layer or power source layer of solid copper leaf interposed between the signal layers. Insulating layers of polyimide or the like are sandwiched between these conductor layers. At least one of the signal layers is formed having a large number of pads for mounting electronic components.
In the case where the mounted electronic components are semiconductor packages of the ball-grid-array type or chip-size packages that are provided with a large number of electrodes, for example, a large number of connecting pads are formed in close order on mounting regions of each printed wiring board. Due to spatial restrictions, it is difficult to wire these numerous connecting pads by means of one signal layer.
Accordingly, some of the connecting pads are connected to another signal layer through a plurality of plated through holes, and are wired by means of this second signal layer. These through holes extend through an intermediate layer, such as the ground or power source layer, and cause the signal layers to conduct to each other. To avoid conducting to the plated through holes, those portions of the ground or power source layer which surround the through holes are cleared of the copper leaf for a given width.
As the printed wiring boards constructed in this manner are reduced in size and improved in packing density, however, the plated through holes are increased in number, and the intervals between the plated through holes are shortened. Thereupon, the area of those regions of the ground or power source layer from which the copper leaf is removed increases, so that these clear regions sometimes may conduct to one another.
In such a case, the function of the ground or power source layer is too low to ensure satisfactory ground connection or good power supply to the mounted electronic components.